1. Field of the Invention
The present invention relates to an image formation controlling method comprising a step of calibrating (adjusting) an image density sensor for detecting the image density of a toner pattern image of each color, which is formed on a photoconductor, an intermediate transfer member or a transfer member of an image forming apparatus such as a copying machine or a printer of various kinds, and to an image forming apparatus used for performing the image formation controlling method.
2. Description of Related Art
Since a photo type image forming apparatus forms an image using electrostatic force, the characteristic thereof is changeable in response to a change in conditions of each processing unit in use and conditions of surroundings. A change in characteristic may cause a change in the density of a formed image and deterioration of image quality. In order to avoid such a situation and to adjust image quality so as to keep obtaining preferable image quality, an image formation controlling step (a process control mode) is provided for adjusting (performing process control on) conditions for image formation (such as a charging output, a light exposure, a development bias voltage, a transfer bias voltage and the amount of toner supply (when two-component developer is used)) of each part of an image forming section.
The quality of an image is adjusted by forming a toner pattern image for a test on an image carrier such as a photoconductor, an intermediate transfer member (an intermediate transfer drum or an intermediate transfer belt) or a transfer member (a transfer drum or a transfer belt) at predetermined timing which is not involved in image formation, measuring the image density of the toner pattern image by an image density sensor configured as an optical sensor and controlling the above-mentioned conditions for image formation by feedback control based on the measured value.
The image density sensor has a light emitting device and a light receiving device and is constructed to emit light from the light emitting device toward the toner pattern image and detect light reflected at the toner pattern image by the light receiving device.
In recent years, an apparatus for forming a chromatic toner pattern image on the surface of an image carrier has been widely used. In such an image forming apparatus, a chromatic test toner pattern image is formed on the surface of an image carrier by a developing device and the amount of adhering toner is detected by an image density sensor. For detecting the amount of adhering toner precisely, a method for detecting a quantity of light diffusely reflected at a test toner pattern image has been proposed (in Japanese Patent Application Laid-Open No. 2000-122360, No. 2001-100481, No. 2002-169345, No. 2-256076 (1990), No. 3-92874 (1991) and the like). Different methods are used for measuring the density of a black (achromatic) toner pattern image and for measuring the density of a chromatic toner pattern image. The density of an achromatic toner pattern image is measured by measuring light (specular-reflection light) specularly reflected at a toner pattern image while the density of a chromatic toner pattern image is measured by measuring diffuse-reflection light as described above.
Since an image density sensor for a toner pattern image detects the quantity of light, which is emitted from a light emitting device and reflected at a toner pattern image, with a light receiving device, it is required to calibrate an optical characteristic of the image density sensor. That is, the amount of adhering toner cannot be detected precisely without calibrating the output of the image density sensor on the basis of a reference value corresponding to a constant amount of adhering toner.
Consequently, accurate calibration of an image density sensor using a jig or the like is performed at the time of manufacturing or assembling and in shipment.
On replacement of a sensor due to an end of the life of a sensor, a malfunction or the like, accurate calibration in the same manner is required again.
However, when a jig comprising a calibration plate for calibration is used, the jig has to be attached with high accuracy. Moreover, since the image density sensor is arranged in a considerably narrow space, it is difficult to attach a large-scale jig which requires accurate positioning. Such a state causes an increase in maintenance cost. Furthermore, on the assumption that user maintenance is performed, it is extremely difficult to perform accurate calibration in view of complication of a calibration operation.
In order to solve the above problems, a method is proposed to dispose a slidable calibration plate, so that the calibration plate is positioned to oppose a sensor when the sensor requires calibration while the calibration plate which is not in use is held back. There is, however, a problem that structure becomes complex because of installment of a sliding mechanism of the calibration plate. There is another problem that it is difficult to avoid contamination by toner due to long use in the image forming apparatus, which cause increase in maintenance cost.